This invention pertains to the field of bar code scanners, and in particular to extending the depth of field for a laser bar code scanner.
Many industries, including the assembly processing, grocery and food processing industries, utilize an identification system in which the products are marked with a bar code symbol consisting of a series of lines and spaces of varying widths, or other types of symbols consisting of series of contrasting markings. A number of different bar code readers and laser scanning systems have been developed to decode the symbol pattern to a multiple digit representation for inventory, production tracking, and for check out or sales purposes. Optical scanners are available in a variety of configurations, some of which are built into a fixed scanning station and others of which are portable. The portability of an optical scanner provides a number of advantages, including the ability to inventory products on shelves and to track portable items such as files, documents, or small equipment. A number of portable scanners (usually moving beam) use lasers which permit the user to scan the bar code symbols at variable distances from the surface on which the bar code is imprinted. However, the possible distances are limited by the depth of field (DOF).
Various optical readers and optical scanning systems have been developed for reading bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia composed of a series of bars of various widths spaced apart from one another to form boundary spaces of various widths, with the bars and spaces having different light reflecting characteristics. The readers and scanning systems transform the graphic indicia information patterns into electrical signals, which are decoded into alphanumerical characters with information content. Such characters are typically represented in digital form and used as input to a data processing system for applications in point-of-sale processing, inventory control, etc.
Bar code symbols are formed from bars or elements that are typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the symbology used. The relative width of the bars and spaces is determined by the type of symbology used, and the actual size of the bars and spaces is usually determined by the application. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol. To encode a desired sequence of characters, groups of elements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies a unique xe2x80x9cstartxe2x80x9d and xe2x80x9cstopxe2x80x9d character is used to indicate where the bar code pattern begins and ends. A number of different bar code symbologies exist. These symbologies include, e.g., PD417, UPC/EAN, Code 39, Code 49, Code 128, Codabar, and Interleaved 2 of 5, etc.
One embodiment of such a scanning system resides in a hand-held, portable laser scanning head supported by a user which is configured to allow the user to aim the light beam at a symbol to be read. The light source in a laser scanner is typically a gas laser or semiconductor laser. The use of semiconductor devices such as a laser diode as the light source in scanning systems is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically modified, typically by a condenser lens, to form a waist-shaped beam in which the width of the beam diminishes with distance until reaching a minimum, or waist, and then increases. The beam is focused so that a desired spot size is achieved at the target (bar code) distance, typically so that the waist is located at the target distance.
The spot size at the target distance should be approximately the same as the minimum width between regions of different light reflectivity, i.e., the width of the bars and spaces of the symbol.
In the scanning systems known in the art, the light beam is directed by a lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. The scanner functions by repetitively scanning the light beam in a line or series of lines across the symbol. The scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the scanner, or do both. Scanning systems also include a sensor or photodetector which functions to detect light reflected from the symbol. The photo-detector is positioned in the scanner or in an optical path in which it has a field of view which extends across and slightly past the symbol. A portion of the reflected light which is reflected off the symbol is detected and converted into an electrical signal. Electronic circuitry or software decodes the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal from the photodetector may be converted into a pulse width modulated digital signal, with the time intervals proportional to the physical widths of the bars and spaces. Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol.
Bar code symbols are printed in varying densities. High density symbols (bar code element widths  less than 0.007 inch) are, for example, used for small parts (e.g., integrated circuits) and for symbols with high information density. Low density symbols (bar code element widths  greater than 0.020 inch) are, for example, used for coding packages and containers in warehouses. As it is generally preferred that the beam scanned across the bar code symbol have a width comparable to that of the minimum width between regions of different light reflectivity (e.g., the minimum width of a bar element), different beam widths are needed to read different density bar codes. Furthermore, bar codes of the same density can be located at varying distances from the laser scanning head.
Conventional laser scanners have a condenser lens that focuses the laser beam so that the spot size is correct at the range at which the bar code reader is expected to operate. With such fixed focus systems, there is typically a xe2x80x9cdead zonexe2x80x9d in front of the scanner in which the spot size is too large for proper operation. Also, such scanners must be focused at the factory by adjusting the condenser lens along the optical axis while observing the spot size and then permanently setting the position of the lens at the position that achieves the desired size. This step is a relatively costly one, adding to the cost of manufacturing the laser scanner.
Various proposals have been made for improvements over these fixed focus imaging scanners. U.S. Pat. No. 4,920,255 shows a bar code reading system in which the range of the surface bearing the bar code is detected using an ultrasonic ranging system, and the detected range is used to prescribe the setting of the optics focusing a laser beam on the bar code (the output signal from the ultrasonic ranging system drives a stepper motor in the laser focusing optics). U.S. Pat. No. 4,831,275 discloses a variety of means for optically modifying the light reflected from the bar code symbol, to vary the distance at which the symbol is in focus on the photodetector within a bar code reader; the techniques taught include altering the shape of a lens, moving an aperture in the optical path, moving a mirror (or a fiber optic cable), and providing an array of sensors, each effectively focused at a different range. U.S. Pat. No. 4,333,006 discloses the use of a plurality of varying focal length holograms placed on a rotating disk to focus at differing overlapping distance ranges.
A number of proposals have been made to improve the operating depth of field for laser scanners. U.S. Pat. No. 5,723,851 describes a laser scanner incorporating multiple lasers focused for different operating ranges. U.S. Pat. No. 5,302,812 shows a laser scanning head in which the range of the beam waist is varied by moving a condenser lens. U.S. Pat. No. 4,808,804 discloses a number of systems for changing the working distance and/or the beam spot size of a laser beam by the light-transmissive properties of pupils or a movable laser light source.
Obtaining images that are free of errors and distortions introduced by the optical elements that are used in the imaging process has long been a goal of those working with imaging systems. Such systems contemplate the imaging of various kinds of objects, including but not limited to bar code symbols, alphanumeric and non-alphanumeric characters and symbols, and blocks of text. For convenience, all such objects are referred to herein as target objects, symbols, or indicia, whether they include encoded data or not. The errors and distortions introduced by the imaging system include, among others, lens aberrations, such as spherical and chromatic aberrations, misfocus errors resulting from an object being located away from the position of best focus, diffraction effects produced by aperture stops, and the diffusion effect associated with some indicia substrates.
An approach to reducing the magnitude of imaging errors is discussed in xe2x80x9cImprovement in the OTF of a Defocussed Optical System Through the Use of Shaded Aperturesxe2x80x9d, by M. Mino and Y. Okano, Applied Optics, Vol. 10 No. 10, October 1971. This article discusses decreasing the amplitude transmittance gradually from the center of a pupil towards its rim to produce a slightly better image. xe2x80x9cHigh Focal Depth By Apodization and Digital Restorationxe2x80x9d by J. Ojeda-Castaneda et al, Applied Optics, Vol. 27 No. 12, June 1988, discusses the use of an iterative digital restoration algorithm to improve the optical transfer function of a previously apodized optical system. xe2x80x9cZone Plate for Arbitrarily High Focal Depthxe2x80x9d by J. Ojeda-Castaneda et al, SPIE Vol. 1319 Optics in Complex systems (1990) discusses use of a zone plate as an apodizer to increase focal depth. While all of these approaches achieve some improvement in image quality, they all have features that limit their usefulness in particular applications, such as bar code reading.
Another approach to reducing the magnitude of misfocus errors is to include appropriate phase masks in the imaging system. One example of this approach is described in U.S. Patent No. 5,748,371 (Cathey et al.). In this patent, the imaging system comprises a lens or lenses and an opto-electronic image sensor. It also includes a cubic phase mask (CPM) which is located at one of the principal planes of the imaging system, and which modifies the optical transfer function (OTF) of the imaging system in a way that causes it to remain approximately constant over some range of distances that extends in both directions (i.e., towards and away from the lens) from the distance of optimum focus. The intermediate image produced by the image sensor is then digitally post-processed to recover a final image which has a reduced misfocus error. While the image correcting technique described above produces results that are substantially better than the results produced by purely optical means, our efforts to use phase masks in imaging type optical readers using this technique have produced unsatisfactory results.
Briefly stated, the operating depth of field for a bar code scanner, preferably a laser scanner, is increased by placing a cubic phase mask (CPM) in the scanning beam. The masked beam is then scanned and reflected or scattered off a bar code and received by a photodetector. The received signal is then processed to recover the original unperturbed representation of the bar code pattern. The processed signal has an increased depth of field over an unmasked scanner signal.
According to an embodiment of the invention, an imaging system for imaging an object located in a target region includes an illumination source; an optical path between the illumination source and the object; a first optics assembly in the optical path; phase masking means in the optical path for receiving light and modifying a phase thereof as a function of position within the phase masking means, thereby creating phase modified light having known effects therein; means for traversing the phase modified light across the object in the target region; image sensing means for receiving light reflected from the object and producing an intermediate image signal therefrom; the intermediate image signal having at least a misfocus error that is dependent upon a distance between the first optics assembly and the object; and processing means for correcting for the known effects of the phase modified light to produce a final image signal of the object. The final image thus has a reduced amount of out of focus error over a range of distances.
According to an embodiment of the invention, a method of scanning for a laser scanning system suited for reading indicia located in a target region, including the steps of (a) generating an illumination beam for illuminating an indicia located in an operational depth of field; (b) changing locally a phase of the illumination beam as a function of position before the beam illuminates the indicia; (c) changing locally an amplitude of the illumination beam as a function of position before the beam illuminates the indicia; (d) receiving light reflected from the indicia; (e) converting the received light to an intermediate image signal; and (f) processing the intermediate image signal such that the operational depth of field is extended.
According to an embodiment of the invention, a method of modifying a laser beam to maximize the system resolving ability as a bar code indicia is moved throughout an operational depth of field includes the steps of (a) generating a laser illumination beam; (b) modifying a phase of the illumination beam a function of position in the beam; (c) modifying an amplitude of the illumination beam as a function of position in the beam; (d) scanning the phase and amplitude-modified beam across the bar code indicia; (e) receiving light reflected from the bar code indicia; (f) converting the received light to an intermediate image signal; and (g) processing the intermediate image signal using a precalculated recovery function to reduce effects of a position of the bar code indicia.
According to an embodiment of the invention, a method of scanning a barcode indicia, the indicia containing information encoded therein, includes the steps of (a) illuminating the indicia in an operating range with light; (b) modifying an amplitude and phase of the light to reduce variations in a localized illumination distribution of the light over the operating range before the light illuminates the indicia; (d) directing the light to the operating range; (e) scanning the light across the indicia; (f) receiving light scattered from the indicia; (g) converting the scattered light to an intermediate image signal; and (h) processing the intermediate image signal to recover the information encoded in the indicia.
According to an embodiment of the invention, a method of increasing an operating depth of field includes (a) providing illumination; (b) distorting a phase of the illumination in a predetermined manner at a specific distance from an object; (c) directing the distorted illumination to the object; (d) receiving light scattered from the object; (e) converting the received light to an intermediate image signal; and (f) processing the intermediate image signal using a precalculated recovery function to reduce effects of the specific distance.